Nociceptin/Orphanin FQ Receptor Structure, Signaling, Ligands, Functions, and Interactions with Opioid Systems

Abstract

The NOP receptor (nociceptin/orphanin FQ opioid peptide receptor) is the most recently discovered member of the opioid receptor family and, together with its endogenous ligand, N/OFQ, make up the fourth members of the opioid receptor and opioid peptide family. Because of its more recent discovery, an understanding of the cellular and behavioral actions induced by NOP receptor activation are less well developed than for the other members of the opioid receptor family. All of these factors are important because NOP receptor activation has a clear modulatory role on mu opioid receptor-mediated actions and thereby affects opioid analgesia, tolerance development, and reward. In addition to opioid modulatory actions, NOP receptor activation has important effects on motor function and other physiologic processes. This review discusses how NOP pharmacology intersects, contrasts, and interacts with the mu opioid receptor in terms of tertiary structure and mechanism of receptor activation; location of receptors in the central nervous system; mechanisms of desensitization and downregulation; cellular actions; intracellular signal transduction pathways; and behavioral actions with respect to analgesia, tolerance, dependence, and reward. This is followed by a discussion of the agonists and antagonists that have most contributed to our current knowledge. Because NOP receptors are highly expressed in brain and spinal cord and NOP receptor activation sometimes synergizes with mu receptor-mediated actions and sometimes opposes them, an understanding of NOP receptor pharmacology in the context of these interactions with the opioid receptors will be crucial to the development of novel therapeutics that engage the NOP receptor.

Fig. 1.

Molecular model of the NOP receptor crystal structure bound to NOP antagonist C-24 (green) (PDB ID: 4EA3). The TM helices are colored in 7 different colors and labeled. The ECL2 loop, between TM4 and TM5 is shown in green. Side chains of amino acids interacting with the antagonist are shown as sticks and labeled.

Fig. 2.

N/OFQ (1-13) peptide (green sticks) bound to the active-state homology model of the NOP receptor. The TM helices are in different colors. The side chains of amino acids interacting with the peptide are labeled. Note the acidic residues of the ECL2 loop (D195, E196) interacting with the basic residues (8-13) of N/OFQ.

Fig. 3.

NOP agonist Ro 64-6198 (green sticks) bound to the active-state NOP receptor model. The small-molecule NOP agonist interacts with the T305 (orange sticks) and Y309 (blue sticks). The phenalenyl group of the NOP agonist is in close proximity to V279 (orange sticks, labeled) within the transmembrane pocket. This residue is isoleucine in the other opioid receptors, which is likely responsible for the lower affinity of Ro 64-6198 for the other opioid receptors.

Fig. 4.

NOP-eGFP receptors are highly distributed in laminae I-III and ×. Tissue sections from the spinal cord were incubated with anti-GFP, and –CGRP (laminae I and IIo, panel A). Tissues were also treated with biotinylated IB4 (dorsal border of lamina IIi) and streptavidin. This figure is reprinted with permission from the Journal of Neuroscience.

Fig. 5.

Colocalization of NOP-eGFP and mu receptors in DRG neurons. Tissue sections were incubated with anti-GFP (green), anti–mu-receptor (red), and anti-NF200 (blue) antibodies. White arrows show small diameter NOP-eGFP+, Mu+ cells. Scale bars 100 μm. This figure is reprinted with permission from the Journal of Neuroscience (Ozawa et al., 2015).

Fig. 6.

Summary of NOP receptor signaling. Figure cartoons the basic NOP receptor signal transduction and trafficking pathways highlighted in this review, and those that have generally been shown by multiple studies. Figure shows NOP receptor canonical coupling to inhibition of calcium channels, and activation of inward rectifying potassium channels. Figure also highlights recent work showing NOP receptor activation of MAPKs, and desensitization pathways via GRK3 and GRK2, and recent data showing that NOP receptors can both positivity and negatively influence cytokine/inflammatory pathway signaling. Furthermore, the cartoon depicts recent papers showing that NOP receptor activation and arrestin signaling can initiate downstream signaling to JNK and ROCK pathways. Arrows refer to activation steps; T lines refer to blockade or inhibition of function.

Fig. 7.

Schematic representation of the organization of basal ganglia regulating motor function and the effects dopamine (DA) depletion on N/OFQ expression and release. DA neurons are represented in blue; glutamate (Glu) neurons in green; GABA neurons in red; color density indicates the relative levels of activity in each system with normal DA neuron function (Panel A, normal function), or after loss of a significant fraction of DA neurons (Panel B). GP, globus pallidus; N/OFQ, nociception/ orphanin FQ; SNc, substantia nigra compacta; SNr, substantia nigra reticulate; STN, subthalamic nucleus, Panel A. With DA neuron function intact, GABA release in the pallido-subthalamic neurons in the “indirect” striato-nigral pathway reduces Glu release from the subthalamic neurons that activate the GABAergic nigrothalamic pathway. With low release of GABA in the thalamus from this pathway, the thalamocortical glutamatergic neurons are active, increasing activity in motor cortex and maintaining normal motor function. N/OFQ levels and release in the SNr are relatively low under these conditions. Panel B. When nigrostriatal DA function is impaired (e.g., after 6-OHDA or MPTP treatment), activity in the subthalamic glutamatergic neurons to the SNr is increased, resulting in activation of the nigrothalamic GABA pathway and inhibition of thalamocortical neurons that facilitate normal motor function. After 6-OHDA or MPTP treatment, ppN/OFQ mRNA and N/OFQ levels and release in SNr are increased (Marti et al., 2005, 2010); N/OFQ release in SNr is also increased by haloperidol treatment (Marti et al., 2010). NOPr antagonists largely reverse the effects of DA depletion by 6-OHDA on GABA release in SNr and thalamus (Marti et al., 2005, 2007, 2010). Treatment with 6-OHDA also reduces NOPr mRNA expression in SNc (Norton et al., 2002; Marti et al., 2005). Arrows indicate the direction of change in N/OFQ, GABA or Glu release after 6-OHDA or MPTP treatment.

Fig. 8.

Chemical structure of non-peptide NOP selective ligands